79 lines
2.9 KiB
Markdown
79 lines
2.9 KiB
Markdown
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---
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title: Declare Variables
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---
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# Declare Variables
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JavaScript variable declarations can be sorted into three distinct components: the variable type, the variable name, and the variable value.
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```js
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var myName = "Rafael";
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```
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Let's break the above line of code into the pieces that make it up:
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```js
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var/const/let
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```
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JavaScript variables can have three declaration types: var, const, and let. Var-type variables are global, if declared outside a function they can be accessed by any JS file (or the console), and if created within a function they are accessible regardless of block scope. Let-type variables are limited in scope to their block. See the example below for the difference.
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```js
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function varTest() {
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var x = 1;
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if (true) {
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var x = 2; // same variable!
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console.log(x); // 2
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}
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console.log(x); // 2
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}
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function letTest() {
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let x = 1;
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if (true) {
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let x = 2; // different variable
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console.log(x); // 2
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}
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console.log(x); // 1
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}
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```
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Const-type variables have the same scope as let variables (block scope), but are immutable. Whatever value a const-type variable is to be assigned, must happen when the variable is declared, and JavaScript will thrown an error if the variable is changed later.
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```js
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const genre = "non-fiction";
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console.log(genre); // "non-fiction";
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genre = "fantasy"; // error
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```
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Now that we can determine what the variable type is, let's take a look at the name. JavaScript variable names are written in `camel case` format. An example of camel case is: `camelCase`. In the context of our example:
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```js
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myName
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```
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The name is also we'll access the variable again later:
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```js
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console.log(myName); // "Rafael"
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```
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Finally, our value:
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```js
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"Rafael"
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```
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JavaScript is dynamically typed, which means any given variable can represent any given data type at any given time. For example:
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```js
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var example = "This is an example";
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example = [0, 1, 2, 3]
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example = {test: "Result"}
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example = 5
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```
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All those statements are perfectly valid - JavaScript variables can jump from string to array to object to integer.
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### Declare object as const
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As mentioned above, const variable are immutable means value assigned to such variable at the time of declaration cannot be updated but there is a point to note in case object declaration with const. Object of type const also cannot be updated once defined but properties of object cab be. For example.
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```js
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const Car1 = {
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name: 'BMW',
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model: 'X1',
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color: 'black'
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}
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```
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Here, we cannot update the object but we can update the properties by accessing through dot(.) operator as below.
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```js
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Car1.color = 'Red';
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console.log(Car1);
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O/P - {name: "BMW", model: "X1", color: "Red"}
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```
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If we need to to make enitre object as immutable (including properties) then we have to use freeze method.
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